Stereo presence signal for an AM stereo system

Abstract

In an AM broadcast transmitter, an infrasonic tone is added to the difference channel of a compatible AM stereo system to provide a stereo presence signal. In a stereophonic receiver, the corrected output of a synchronous detector in the difference signal channel is coupled to an infrasonic detector for control of a mono/stereo mode switch and a stereo presence indicator. When the received signal is weak, the infrasonic tone is effectively stronger, because the phase angle is increased. Since the s/n ratio is proportional to the phase angle, the stereo presence signal is self-adjusting. The mode switch and indicator are enabled for all values of L and R. The infrasonic tone could also be removed from the transmitted signal whenever the difference signal goes above a predetermined level during stereo broadcasting, in which case a comparator at the receiver would enable the mode switch and indicator upon detection of either a difference signal or the infrasonic tone.

What is claimed is: 
     
       1. A communication system wherein signal information corresponding to first (L) and second (R) intelligence signals is transmitted in quadrature and is compatible for both monophonic and stereophonic operation and further including a stereo presence signal, the transmitted signal having the form (1+L+R)cos(ω c  t+φ) where ω c  t is a carrier frequency and φ is arc tan {(L-R+SP)/(1+L+R)═ where SP is the stereo presence signal, the system comprising in combination: transmitter means for generating a single carrier frequency amplitude modulated in accordance with the algebraic addition of said first and second intelligence signals and phase modulated by an angle whose tangent is the ratio of the sum of the stereo presence signal and the difference between the first and second intelligence signals to the envelope of the amplitude modulated carrier; and   receiver means capable of operating in either of two operational modes for receiving said carrier wave and for demodulating said first and second intelligence signals in quadrature for stereophonic operation and for detecting said stereo presence indicator signal for controlling the operational mode of the receiver means.   
     
     
       2. The communication system according to claim 1 wherein the receiver means further includes indicator means enabled by the detection of said stereo presence indicator signal. 
     
     
       3. A communication system according to claim 1 wherein the transmitter means further includes control means removing the stereo presence signal when the amplitude of the difference signal reaches a predetermined level. 
     
     
       4. A communication system according to claim 3 wherein the receiver means further includes gating means for controlling the operational mode of the receiver means in response to detection of one of the stereo presence signal and the difference signal. 
     
     
       5. A receiver for receiving a broadcast carrier wave which is amplitude modulated with signal information proportional to the sum of first (A) and second (B) intelligence signals and which is phase modulated by an angle φ having a form φ=arc tan C 1  (A-B+SP)/(C 2  +A+B) where C 1  and C 2  are constants and SP is the amplitude of a stereo presence indicator signal, the receiver comprising in combination: means for selectively receiving the modulated carrier wave;   means for translating the received carrier wave to one of an intermediate frequency;   corrector circuitry for providing output signals which are substantially equal to the first and second intelligence signals;   detector means for detecting the stereo presence indicator signal;   switching means movable between first and second positions for enabling receiver operation in monophonic and stereophonic modes respectively in response to the detection of said stereophonic presence indicator signal.   
     
     
       6. The receiver according to claim 5 and further including an indicator means coupled for being enabled by the detection of the stereo presence indicator signal. 
     
     
       7. A receiver for receiving a compatible AM stereo signal including an infrasonic stereo presence signal having an amplitude SP, the received signal having the form (1+L+R)cos(ω c  t+φ) where L and R represent first and second intelligence signals, ω c  t is a carrier frequency and φ is arc tan {(L-R+SP)/(1+L+R)} the receiver comprising in combination: receiving means for providing an intermediate frequency signal in response to the received signal;   envelope detector means coupled to the receiving means for providing an output proportional to L+R;   synchronous detector means coupled to the receiving means for providing an output proportiona to (L-R+SP)cos φ;   corrector means for providing a correction signal proportional to cos φ;   divider means coupled to the synchronous detector means and to the corrector means for providing an output signal proportional to L-R+SP;   matrixing means coupled to the envelope detector means and to the divider means for providing signals proportional to L and R;   detector means for detecting the presence of the stereo presence signal; and   switching means coupled to the detector means for switching the receiver mode in response to detection of the stereo presence signal.   
     
     
       8. The receiver in accordance with claim 5 and further including indicator means coupled to the detector means for providing indication of the presence of the stereo presence signal. 
     
     
       9. A receiver for receiving a compatible AM stereo signal amplitude modulated with 1+L+R and phase modulated with an angle φ=arc tan {Q/(1+L+R)} where L and R represent first and second intelligence signals and Q is alternatively (L-R) or SP where SP is the amplitude of an infrasonic stereo presence signal, the receiver including in combination: receiving means for providing an intermediate frequency signal in response to the received signal;   envelope detector means coupled to the receiving means for providing an output proportional to L+R;   synchronous detector means coupled to the receiving means for providing an output proportional to Q cos φ;   corrector means for providing a correction signal proportional to cos φ;   divider means coupled to the synchronous detector means and to the corrector means for providing an output proportional to Q;   matrixing means coupled to the envelope detector means and to the divider means for providing signals proportional to L and R;   detector means for detecting the presence of the stereo presence signal;   gating means coupled to the divider means and to the stereo presence signal detector means for providing a control signal in response to the detection of one of the L-R and stereo presence signals; and   switching means coupled to the detector means for switching the receiver mode in response to the control signal.   
     
     
       10. The receiver in accordance with claim 9 and further including indicator means coupled to provide indication of the presence of one of the stereo presence signal and the L-R signal.